Domain Modeling Made Functional

Modeling an Order as a Set of States

Now let’s move on to the steps in the workflow pipeline. It’s clear from our previous understanding of the workflow that the Order isn’t just a static document but actually transitions through a series of different states:

How should we model these states? A naïve approach would be to create a single record type that captures all the different states with flags, like this:

type Order = {
  OrderId : OrderId
  ...
  IsValidated : bool  // set when validated
  IsPriced : bool     // set when priced
  AmountToBill : decimal option // also set when priced
  }

But this approach has lots of problems:

• The ...